Is paramutation coupled to developmental processes which determine tassel induction?

--Bernard C. Mikula

The L:D treatments above shift the corn plant from the vegetative to the flowering phase of development. Plant physiologists have programmed plant growth and development with day length since the l920's.
Fig. 5 shows the differences in plant habit (inbred W22) which resulted from the application of the L:D treatments to seedlings. The plant on the left was raised in constant light at 22 C until day 15 then given L:D treatments at 22 C days 16-21. The plant on the right was raised in constant light until day 10 at 32 C then shifted L:D conditions at 32 C from days 11-15. Both plants look essentially alike in habit though the one from the higher temperature was determined six days earlier. The middle plant was grown for 10 days in constant light at 32 C then transferred to 22 C in continuous light from days 11-15. All plants were transplanted to field conditions at the end of their respective treatments. The developmental effects of each of the treatments can be seen in the photographs. The plant on the left had the fewest tassel branches. Larger numbers of tassel branches were found on plants given higher seedling temperatures. Four to five times as many tassel branches were found on the plants which received the higher temperature in continuous light, of which the middle plant is a typical representative. An average of three more nodes accounts for their being taller with anthesis a week later. Along with these morphological changes programmed by temperature and day length, the variation in the level of paramutation discussed above is influenced at this same developmental period.

Paramutation provides a model system where it is possible to follow incremental change in the expression of a single gene across generations. Thus, environmental programming of gene expression becomes an experimental possibility, especially since the paramutant R gene has been shown to have an additive memory capability from generation to generation. What molecular transducers respond to temperature and light? Where and how is the incremental memory stored from generation to generation? The answer to these questions can begin to explain how native plants have been able to respond to changing glacial boundaries across continental latitudes and altitudinal climatic boundaries. If a genetic feedback from environmental conditions exists, then it would seem appropriate that it be coupled to the mechanisms associated with control of reproductive physiology known to be entrained by day length conditions.

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